Conventional flash memory with a memory cell that stores charge in a charge storage layer as is used widely. This is because this type of memory has a cost per bit is low, there is a high level of integration, and the memory is non-volatile. For example, NAND-type flash memory can be highly integrated, as the connections between memory cells can be reduced by connecting memory cells in series between selected transistors.
Relatively recently, multi-value storage systems that can store more than 2 bits of data in a single memory cell have emerged in competition with conventional systems, which store only 2 bits per memory cell. These devices have increased capacity and, generally, are cheaper to manufacture and smaller in size.
In a data write operation (program operation) of NAND-type flash memory, a verify read operation is performed to determine if the target threshold voltage has been met. If the number of cells which have not met the target threshold voltage (verify read voltage) level is greater than a predetermined number, the write voltage is increased (this is called a step-up operation). Subsequent verify read operations are repeated until the target threshold voltage is finally obtained.
It is well known that if the write operation and the erase operation in the NAND-type flash memory are repeated many times there will be a gradual deterioration of the gate-insulating film of the memory cell, which may reduce the data retention characteristics of the memory cell. As data retention characteristics deteriorate, there will be an increase in the probability of a data misread.
In addition, it would be preferable to shorten the write time of NAND-type flash memory devices to improve their performance.